1. Field of the Invention
The present invention relates to an optical-element supporting device for supporting an optical element and, more particularly, to an apparatus for coupling a mirror or lens to a lens barrel of an optical projecting system of a semiconductor device.
2. Description of the Related Art
A reduction projection exposure apparatus for projecting a circuit pattern drawn on a reticle onto a wafer or other components using an optical projecting system and transferring the circuit pattern thereto is used in fabricating a microdevice employing photolithography.
A reduction projection exposure apparatus using extreme ultraviolet (EUV) light of approximately 10 nm to 15 nm, which has a shorter wavelength than that of ultraviolet light (hereinafter referred to as “EUV exposure apparatus”) is developed to transfer a very fine line no more than 0.1 μm of a circuit pattern with high resolution.
In an EUV exposure apparatus exposing with high resolution, its optical projecting system is required to have considerably high precision. For the shape of a mirror, distortion of only approximately 1 nm or less is allowable. If an optical element, such as a mirror, is distorted, the optical path changes before and after the distortion, rays to focus do not converge at a point, aberration occurs, image blurring and misregistration occurs, which can result in a short circuit of a circuit pattern on the wafer.
Accordingly, it is necessary to constrain six degrees of freedom of the optical element properly in order to prevent the optical element in the optical projecting system from being distorted and to support the optical element while maintaining a natural frequency high in order to avoid the occurrence of image blurring.
One example of a supporting device for supporting an optical element is disclosed in Japanese Patent Laid-Open No. 2002-350699. The optical-element supporting device supports three sites of an external edge of an optical element using a supporting unit including a plurality of leaf springs.
It is necessary for an optical projecting system of an EUV exposure apparatus to arrange a mirror driving device, a position sensor, and a temperature control device in a limited space around mirrors. In addition, optical design constraints lead to a configuration in which the distance between the mirrors is small and a beam passes in close vicinity to the mirrors. Accordingly, with a structure in which a mirror is supported at three sites, as in a known example, a space for arranging a position sensor and other necessary elements adjacent to the mirror is restricted. Because the space for arranging the position and other elements is restricted, a space for a structure supporting them is also restricted. This may result in difficulty in arranging the position sensor and other elements while maintaining sufficient rigidity. Therefore, it is necessary to support the mirror more compactly in order to arrange the position sensor and other elements around the mirror while maintaining a high rigidity (natural frequency).
Moreover, it is also required that six degrees of freedom of the mirror be constrained properly to protect the mirror from being distorted.